Conventionally, a copying machine incorporating a movable document platen, wherein a document platen for placing thereon a document is moved when an optical system scans the document, has been used. In this type of copying machine, when the optical system scans the document, first, the document platen is moved backward from a stand-by position to a scanning start position. Then, while the scanning is being carried out, the document platen is moved forward until it reaches a scanning end position. Thereafter, the document platen is moved backward from the scanning end position to the original stand-by position.
As shown in FIG. 18, a driving device 151 for moving a document platen 152 is provided under the document platen 152, and has a configuration as shown in FIG. 18. When moving forward (in the direction of arrow F in FIG. 18) the document platen 152 from the scanning start position, a solenoid 153 for forward movement is turned ON. As a result, a latch 154 engages with a crutch brake 156 of a spring crutch 155 for forward movement, thereby actuating the spring crutch 155 for forward movement.
Therefore, the driving force passed to an input gear 157 is first transmitted to an output gear 158, and further transmitted to a platen driving gear 159, thereby moving forward the document platen 152 having a rack which meshes with the platen driving gear 159.
When the platen 152 has reached the scanning end position, a detection switch 160 shown in FIG. 17 detects the arrival of the platen 152 at the scanning end position. Then, the solenoid 153 for forward movement is turned OFF, while a solenoid 161 for backward movement is turned ON. This brings a latch 162 to engage with a clutch brake 163 of a spring clutch 167 for backward movement, thereby actuating the spring clutch 167.
As a result, the driving force passed to the input gear 157 is transmitted to the platen driving gear 159 via a gear 164, an input gear 165, and output gears 166 and 158. Then, the document platen 152 is moved backward (in a direction of arrow R in FIG. 18) to the original stand-by position.
However, with the above-mentioned conventional configuration, in order to achieve reciprocating movement of the platen 152, the spring clutch 155 for forward movement, the spring clutch 167 for backward movement, the solenoid 153 for forward movement, and the solenoid 161 for backward movement, etc., must be provided. In the copying machines incorporating movable document platens, since most of them are compact and economic type, the proportion of these components to the total cost significantly increases, thereby presenting the problem of high manufacturing cost of the copying machine.
The publication of Japanese utility model gazette (Jitsukousho 56-22205) discloses a document platen driving device which includes a loop shaped internal rack, and a pinion which meshes with the internal rack.
However, with the above arrangement of the document platen driving device, the forward moving speed is the same as the backward moving speed. Thus, the above arrangement presents the problem that when carrying out a successive copying process, a long time is required. More concretely, there is a limit in increasing the transport speed and the transfer speed of the sheets in the copying machine in order to ensure the copying quality, and thus there is a limit in increasing the forward moving speed of the document platen. When the forward moving speed of the document platen is the same as the backward moving speed of the document platen, a waiting time for the next copying process becomes long. As a result, a long time is required for the entire copying process.
In order to counteract the above problem, the Laid-open Japanese patent publication (Tokukaisho 57-122458) discloses the document driving device wherein a length of the periphery of a loop-shaped internal rack provided with the document platen can be adjusted according to the document size. With this arrangement, the entire time required for the successive copying process can be reduced for a certain size of the document. However, the complex configuration is required in order to vary the length of the periphery of the rack. Thus, the above arrangement presents the problems that the device becomes larger in size and the manufacturing cost thereof increases. For this reason, in practice, the above document platen driving device cannot be adopted in the compact copying machine.